1. Field of the Invention
This invention relates to a vibration damper for attenuating gear rattle due to torsional vibrations in a power output shaft for a geared transmission.
2. Background Art
In a power transmission mechanism for an automotive vehicle, a vehicle engine delivers driving power to a transmission power output shaft through multiple torque flow paths defined by torque transmitting gearing. In the case of a manually-controlled multiple-ratio transmission, the torque flow paths are established and disestablished by torque delivery transmission synchronizers and an engine clutch with an engine torsional vibration damper. The transmission power output shaft is subjected to high frequency torque disturbances due to the combustion forces of the engine transmitted through the engine clutch and inertia forces developed by the gears at each gear mesh. The forces at a gear mesh to which the power output shaft is subjected are referred to commonly as gear rattle forces.
The meshing engagements of the teeth of individual transmission gear elements, of necessity, have a degree of backlash since a zero backlash gear mesh would result in unacceptable gear tooth forces at the gear tooth surfaces. The gear elements, which typically have involute profiles, require a predetermined backlash condition to compensate for dimensional changes in the gearing geometry due to temperature changes in transmission lubricating fluid during normal vehicle operation. A so-called gear rattle condition is a result of the pre-designed backlash at each gear mesh. The gear rattle resulting from torsional vibration, as well as torsional vibrations resulting from the engine combustion forces transferred through the clutch damper, cause transmission power output shaft torsional vibrations.
The transmission power output shaft has a natural resonant torsional vibration frequency. The rotary speed of the transmission power output shaft at which that resonant frequency occurs depends upon the shaft geometry, the inertia mass of transmission components connected to the shaft and the characteristics of the clutch torsional vibration damper.
In a typical automotive power transmission mechanism in contemporary automotive powertrains, a transmission power output shaft resonant frequency may occur at intermediate transmission output shaft speeds between 1,000 rpm and 2,000 rpm. A typical resonant frequency for a power output shaft in a contemporary automotive transmission may be approximately 68 Hz, which may occur at about 1,375 rpm.
Attempts have been made to attenuate the torsional vibrations in a rotary power delivery shaft in various driveline environments by mounting on the shaft a torsional damper with a natural resonant frequency that can be tuned to counteract the natural resonant frequency of the shaft. An example of a damper of this type is disclosed in U.S. Pat. No. 3,848,694, which is specifically designed for eliminating torsional vibrations resulting from a differential gearing in a differential and axle assembly to which an engine driven driveshaft would be connected in a vehicle driveline. It is intended to attenuate gear noise created by the differential gearing. A gear noise condition, however, is a phenomenon that is different than a gear rattle condition, which is the condition with which the present invention is concerned. The device of the '694 patent would be incapable of attenuating torsional vibrations in a transmission power output shaft caused by gear tooth impact and gear rattle at a gear mesh.
U.S. Pat. No. 4,884,666 is an example of another design approach for using a torsional damper for an automotive vehicle driveshaft at a location in the driveline remote from the transmission itself. The design of the '666 patent is intended to deal with gear noise in an axle rather than gear rattle due to torsional vibrations. The damper of the '666 patent is mounted at a selected location on an automotive driveshaft, which is formed in two pieces. The adjacent ends of each driveshaft piece of that design are secured together by the damper assembly, each adjacent end being welded to opposite sides of the damper assembly.